Unused tags remain in browser list
Removing tags from notes (or notes from your collection) will sometimes leave tags in the left pane of the browser that are no longer used on any cards. If you wish to delete these unused tags, you can do so by choosing Tools → Check Database from the menu in Anki's main window.
For the curious, this is due to performance issues (it would cause significant delays to check every tag in the collection every time a tags or card were deleted).
The sync protocol does not currently cover tag deletions, so if you use Anki on multiple devices, after checking the database, please go to Settings>Preferences>Network and check the box to force a one way sync, choose Upload when syncing to upload to AnkiWeb, and then sync your other devices to download the updated collection to them.